As mobile devices have been increasingly developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as an electric bicycle (E-bike), an electric vehicle (EV), and a hybrid electric vehicle (HEV), as well as an energy source for mobile wireless electronic devices, such as a mobile phone, a digital camera, a personal digital assistant (PDA), a portable multimedia player (PMP), and a laptop computer.
A small-sized battery pack, in which a battery cell is packed, is used for small-sized devices, such as a mobile phone and a digital camera. On the other hand, a middle or large-sized battery pack, in which a battery pack including two or more battery cells (hereinafter, occasionally referred to as a “multi-cell”) connected to each other in parallel and/or in series is packed, is used for middle or large-sized devices, such as a laptop computer and an electric vehicle.
As previously described, a lithium secondary battery exhibits excellent electrochemical properties; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and an electrical short circuit, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused with the result that heat and gas are generated and the high-temperature and high-pressure condition caused by generation of the heat and the gas accelerates the above-mentioned decomposition. Eventually, a fire or explosion may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit to interrupt electric current when the battery is overcharged or overdischarged or when overcurrent flows in the battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the battery increases, and a safety vent to interrupt electric current or exhaust gas when pressure increases due to generation of the gas. In case of a small-sized cylindrical secondary battery, for example, the PTC element and the safety vent are usually disposed at the top of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical container. In case of a small-sized prismatic or pouch-shaped secondary battery, on the other hand, the protection circuit module and the PTC element are usually mounted at the upper end of a prismatic container or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety-related problem of the lithium secondary battery is even more serious for a middle or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells is used in the multi-cell battery pack, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells with the result that a fire or explosion may occur, which may lead to a large-scale accident. For this reason, the middle or large-sized battery pack is provided with a safety system, such as a fuse, a bimetal, and a battery management system (BMS), to protect the battery cells from overcharge, overdischarge, and overcurrent.
However, as the lithium secondary battery is continuously used, i.e. as the lithium secondary battery is repeatedly charged and discharged, the generating element and electrical connection members are gradually degraded. For example, degradation of the generating element leads to decomposition of electrode materials and the electrolyte, by which gas is generated. As a result, the battery cell (the container or the pouch-shaped case) gradually swells. In the normal state of the lithium secondary battery, the safety system, i.e. the BMS, detects overdischarge, overcharge, and overcurrent of the battery pack to control/protect the battery pack. In the abnormal state of the lithium secondary battery, however, when the BMS does not operate, a possibility of the risk increases and it is difficult to control the battery pack in order to secure the safety of the battery pack. The battery pack is generally constructed to have a structure in which a plurality of battery cells is fixedly mounted in a predetermined case. As a result, the respective swelling battery cells are further pressurized in the restrictive case, whereby risk of a fire or explosion greatly increases under an abnormal operating condition of the battery pack.
In connection with this matter, FIG. 1 is a circuit diagram typically showing a conventional battery pack. Referring to FIG. 1, a conventional battery pack 900 includes a battery module assembly 910 including a plurality of battery modules electrically connected to each other, each of the battery modules including a plurality of battery cells or unit modules connected to each other in series while being mounted in a module case, a BMS 920 to detect information regarding an operation state of the battery module assembly 910 and to control the battery module assembly 910 based on the detected information, and a power switch unit (relay) 930 to perform connection or disconnection between the battery module assembly 910 and an external input and output circuit (inverter) 940 according to an operation command from the BMS 920.
The BMS 920 keeps the power switch unit 930 on during a normal operating condition of the battery module assembly 910 and, upon detecting abnormality of the battery module assembly, turns the power switch unit 930 off to stop charge and discharge operations of the battery module assembly 900. During malfunction or non-operation of the BMS 920, on the other hand, the BMS 920 does not perform any control operation with the result that the power switch unit 930 is kept on. Consequently, charge and discharge operations of the battery module assembly 910 are continuously performed even in the abnormal operation state of the battery pack.
Furthermore, in a case in which battery modules are arranged in two or more rows to constitute a battery pack, it is difficult to predict which row of the battery modules will be overcharged.
Therefore, there is a high necessity for technology that is capable of fundamentally securing safety of the battery pack while solving the above problems.
In addition, there is also a necessity for a battery pack of a specific structure in which the battery pack, including battery modules arranged in two or more rows to provide high output and large capacity, is protected from vibration and impact, thereby securing durability of the battery pack, and the battery pack is configured to have a compact structure.